The present invention relates to a method of positioning a workpiece, for example a roughly-cut drill bit, in a predetermined angular position in a machine-tool. It also relates to a device or apparatus for carrying out this method.
Taking a drill bit as an example, the production method consists firstly in producing a roughly-cut drill bit which is finished except for its end or tip which is left in an unfinished state. This tip is then machined in a grinding machine to a conical shape with cutting edges defined by the intersection of two surfaces.
The roughly-cut drill bit, whose diameter is equal to the nominal diameter of the finished drill bit, includes helicoidal or straight flutes for removing chips, the edges of these flutes forming one of the surfaces of the finished cutting edges.
The purpose of sharpening is to shape the other surface of each cutting edge. This is a delicate operation that must precisely be carried out because the angle made by this surface with the other surface of the cutting edge directly influences the quality of the drill bit. It is therefore essential to place the roughly-cut drill bit on the machine in a well determined angular position in order to exactly know the position of the flutes relative to the grinding wheel. This then enables machining of the tip in such a manner that the angle between the two surfaces defining the cutting edges has the desired value.
Angular positioning of the flutes of a roughly-cut drill bit has heretofore been obtained by methods consisting, for example, in bringing the side of a flute into contact with a mechanical sensor. Or, as described in detail in Swiss patent 659 790, a roughly-cut drill bit is placed in a flux of parallel light rays from a lamp, then the intensity of the flux intercepted by the drill bit is measured by means of a photosensitive device, the measured intensity being representative of the angular position of the flutes.
The method employing a sensor has the disadvantage that it necessarily involves a delicate mechanical device of limited precision.
With the optical method disclosed in the citation, only the maximum value of the strength of the intercepted light flux gives a well defined angular position of the flutes of the roughly-cut drill bit. In theory, this maximum is clearly marked, but in practice because of non-parallelism of the light rays and defective symmetry of the roughly-cut drill bit, the maximum is relatively flat and therefore difficult to localize. This difficulty, which limits the positioning precision to about 30 minutes of an arc, is the main drawback of this method.